The recent demonstration that several cloned eukaryotic genes contain DNA stretches which interrupt the coding regions (so called introns) has been a surprise, and as yet the functional significance of these introns is not understood. The proposed experiments aimed at an investigation of the function of these sequences in the rDNA cistrons of Dipteran flies, where they have been characterized and isolated in recombinant DNA plasmids from three species to date (Drosophila melanogaster, Drosophila virilis, and Calliphora erythrocephala). The studies will concentrate on the introns of C. erythrocephala - a species which offers the opportunity to do both cytological and, because of its size, biochemical studies. Structural studies to investigate the homologies between the introns of the three species will establish whether conserved regions, which might therefore have functional significance exist in the introns. In situ hybridization of pure rDNA intron-derived sequences of C. erythrocephala to the polytene chromosomes of this species will establish the other genomic loci for these sequences outside the rDNA itself. The evidence to date indicates that transcription of the intron-containing rDNA cistrons occurs in ovarian germ-line cells, which are highly specialized for ribosome synthesis, and that this may be accompanied by co-ordinate transcription of intron sequences at other genomic loci. A detailed investigation of intron-sequence transcription in ovarian tissue which will probe the possibility of a regulatory role for the intron sequences is a major objective of this proposal.